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In a surprise announcement, Lebanon's Hizbollah militia has blamed the killing of a militant described as its top commander in Syria on extremist Sunni insurgents.

Many expected the powerful Shia group to point a finger at its traditional nemesis, Israel.

Hizbollah revealed on Friday that Mustafa Badreddine, one of its most senior figures, died in a mysterious blast in Damascus, the Syrian capital.

Before leading thousands of militants in Syria, Badreddine, 55, is suspected of having roles in the assassination of a Lebanese prime minister in 2005, and other bombings that date to the attack on the US Marine barracks in Beirut in 1983.

Analysts said Badreddine's killing appeared to bear the hallmarks of an airstrike by Israel, which has targeted a number of the Lebanese militants in Syria in recent years. But Hizbollah blamed it on "artillery bombardment carried out by takfiri groups in the area". Hizbollah uses takfiri, an Arabic word, to describe its extremist Sunni Muslim enemies, including al-Qaeda and Isis (Islamic State).

But the Syrian Observatory for Human Rights, a Britain-based monitoring group, said there has been no shelling for a more than a week in the area where Hizbollah said Badreddine was killed, Reuters reported.

If Hizbollah had blamed Israel for his death, the group would have come under pressure to launch a tough retaliation that, in turn, would risk triggering war. Israel and Hizbollah fought a brief but devastating war in 2006.

In recent weeks, scores of the militants from Iran, Iraq and Hizbollah have been killed by hard-line Sunni groups, notably al-Qaeda-affiliated Jabhat al-Nusra.

Badreddine's slaying by hardline Sunni fighters, if confirmed, would further highlight how the Syrian civil war has become a proxy conflict driven by sectarian divisions.

Saudi Arabia, a Sunni powerhouse, plays an important role in backing the Sunni-led rebellion that is fighting Hizbollah and other pro-government Shia fighters that have loyalties to Iran.

Saudi Arabia's primary enemy is Iran, a Shia nation, and the two countries are locked in a region-wide competition for influence.

"Things will escalate because of this," said Talal Atrissi, a Lebanese analyst who is close to Hizbollah. "I expect that in retaliation for Badreddine's killing, Hizbollah will carry out a number of special operations attacks against Jabhat al-Nusra and Isis." In recent weeks, a shaky ceasefire that took effect in February appears to have intensified battles between pro-government Shia militants and hard-line Sunnis, particularly Jabhat al-Nusra. The al-Qaeda affiliate is not party to the ceasefire, allowing it to carry out more robust assaults against pro-government forces.

Last week, Jabhat al-Nusra fighters attacked a government-held area near the northern city of Aleppo, killing scores of Iranian and apparently Hizbollah fighters. A tally of media reports on the killings by Reuters put the number of dead at as high as 80.

At least 17 of those killed were Iranians, which the news agency said could have been the highest toll in a battle outside the Islamic Republic since its war with Iraq in the 1980s.

Since the start of the Syrian conflict in 2011, dozens of Iranian militants - including generals - have been killed in Syria. Some estimates put the number of Hizbollah dead at well over 1000. Iran and Hizbollah intervened militarily to prop up the Syrian leader, their ally.